The present invention relates to a method for producing a flow rich in methane and a flow rich in C2+ hydrocarbons from a supply flow containing hydrocarbons, of the type comprising the following steps:                cooling at least a first fraction of the supply flow in a first heat exchanger;        introducing the first cooled supply fraction into a first separation flask in order to produce a light upper flow and a heavy lower flow;        dividing the light upper flow into a turbine supply fraction and a column supply fraction;        pressure reduction of the turbine supply fraction in a first dynamic pressure reduction turbine and introduction of at least a portion of the fraction subjected to pressure reduction into the first turbine in a middle portion of a first distillation column;        cooling and at least partially condensing the column supply fraction in a second heat exchanger, pressure reduction and introduction of the cooled column supply fraction into an upper portion of the first distillation column;        pressure reduction and partial vaporisation of the heavy lower flow in the first heat exchanger and introduction of the heavy lower flow subjected to pressure reduction into a second separation flask in order to produce an upper gas fraction and a lower liquid fraction;        pressure reduction of the lower liquid fraction and introduction in the middle portion of the first distillation column;        cooling and at least partially condensing of the upper gas fraction in the second heat exchanger and introduction into the upper portion of the first distillation column;        recovery of a lower column flow at the bottom of the first distillation column, the flow rich in C2+ hydrocarbons being formed from the lower column flow;        recovery and reheating of an upper column flow rich in methane,        compressing at least a fraction of the upper column flow in at least a first compressor coupled to the first dynamic pressure reduction turbine and in at least a second compressor;        forming the flow rich in methane from the reheated and compressed upper column flow;        removing an extraction flow from the upper column flow;        cooling and introducing the cooled extraction flow into an upper portion of the first distillation column.        
Such a method is intended to extract C2+ hydrocarbons, such as in particular ethylene, ethane, propylene, propane and heavier hydrocarbons, particularly from natural gas, refinery gas or synthetic gas obtained from other hydrocarbon sources, such as carbon, crude oil, naphtha.
Natural gas generally contains a majority of methane and ethane constituting at least 50 mol % of the gas. It also contains, in a more negligible quantity, heavier hydrocarbons, such as propane, butane, pentane. In some cases, it also contains helium, hydrogen, nitrogen and carbon dioxide.
It is necessary to separate the heavy hydrocarbons from the natural gas in order to comply with at least two imperatives. Firstly, economically, C2+ hydrocarbons and particularly ethane, propane and butane have a worth which can be exploited. Furthermore, the demand for liquid natural gas as a charge for the petrochemical industry increases continuously and should continue to increase in the years ahead.
Furthermore, for processing reasons, it is desirable to separate the heavy hydrocarbons in order to prevent them from condensing during transport and/or handling of the gases. This allows prevention of incidents such as the occurrence of liquid plugs in transport or processing installations configured for gaseous effluents.
In order to separate the C2+ hydrocarbons from the natural gas, it is known to use an oil absorption method which allows recovery of up to 90% of the propane and up to approximately 40% of the ethane.
In order to achieve higher recovery rates, cryogenic expansion methods are used.
In a known cryogenic expansion method, a portion of the supply flow containing the hydrocarbons is used for the secondary distillers of a methane separation column. Subsequently, the various effluents, after partial condensation, are combined in order to supply a gas/liquid separator.
As described in U.S. Pat. No. 5,555,748, the light flow obtained in the upper portion of the separator is divided into a first column supply fraction, which is condensed before being conveyed to the upper supply of the distillation column and a second fraction which is conveyed to a dynamic pressure reduction turbine before being reintroduced into the distillation column.
This method has the advantage of being easy to start and of providing substantial operating flexibility, combined with good efficiency and good reliability.
However, economic constraints require a further increase in the efficiency of the method whilst retaining a very high level of efficiency of ethane extraction. It is further necessary to minimise the spatial requirement of the installations and to reduce, or even dispense with, the provision of external coolants such as propane, in particular for carrying out the method on floating installations or in zones which are sensitive in terms of security.